pNFS/flexfiles: Report ENETDOWN as a connection error

[ Upstream commit aa42add73ce9b9e3714723d385c254b75814e335 ]

If the client should see an ENETDOWN when trying to connect to the data
server, it might still be able to talk to the metadata server through
another NIC. If so, report the error.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>